1. Field of the Invention
The present invention relates generally to knee prosthesis and, more specifically, to hinged knee prosthesis that achieves better emulation of natural knee joint kinematics in a prosthetic knee.
2. Brief Description of the Prior Art
During articulation of a natural knee joint, flexion between the tibia and the femur takes place about a transverse axis while, at the same time, some relative rotation between the tibia and the femur occurs about a longitudinal axis. Such flexion and rotation is necessary to carry out a normal gate cycle. It has been established that in full extension the tibia is rotationally displaced, relative to the femur, by approximately 2-3 degree. As the natural knee flexes, the tibia rotates internally. According to previous studies, about 5 degree of rotation ordinarily occurs as the knee is articulated from 0 degree to 10 degree of flexion; thereafter, little further rotation occurs up to at least about 45 degree of flexion. Total rotation at 110 degree of flexion is approximately 20 degree.
Rotational stability of the natural knee is provided by the collateral and cruciate ligaments. The cruciate ligaments deter uncontrolled internal rotation within a certain range of flexion of the knee, while the collateral ligaments provide transverse stability and deter uncontrolled external rotation of the tibia. Where the natural knee is replaced by a total knee prosthesis, either the anterior cruciate ligament or both the anterior and posterior cruciate ligaments ordinarily are sacrificed. In the instances where the knee prosthesis is constrained to supply the stability ordinarily provided by the sacrificed ligaments it is desirable for the knee prosthesis to mimic the natural knee as closely as possible.
Early hinged knee designs had a fixed hinge connecting the tibial and femoral components. This hinge had a fixed center of rotation, and as a result any “natural” rotations and translations that may have occurred between the femur and tibia during flexion of the knee manifested themselves as forces at the bone-implant interface. Due to these forces, early hinged knee designs tended to fail by becoming loose at the bone-implant interface.
Subsequently improvements in hinged knee designs have resulted in the hinge being associated with a circular post that is inserted into the tibial component. The effect of this post is that the hinge no longer has a fixed center of rotation. A hinge on a circular post is able to both translate along one axis and rotate about that axis. This design results in lower forces being transmitted to the bone-implant interface during knee flexion, as a result increasing the longevity of hinged total knee replacements. However, any anterior-posterior translation of the center of rotation is prevented. Since the movement of the femoral component of these hinged knee designs has a translational component in anterior-posterior direction with respect to the tibial component, forces are generated that act to weaken the bone-implant interface. The generation of these forces is avoided in non-hinged total knee replacement prosthesis because the femoral component and the tibial component are not rigidly connected via a hinge.
Non-hinged total knee replacements rely on the knee ligaments and other soft tissues to provide stability to the knee joint and in part dictate the relative motion between the femoral and tibial components. As a result of the soft tissue guiding the relative motion of the knee between the femoral and tibial components, the knee moves in a more “natural” manner than can be provided by existing hinged knee designs. Extra amount of stability can be provided by a post on the tibial component interfacing with a cam on the femoral component during knee flexion. Unlike a hinge, the post and cam are not connected to each other in a fixed manner and therefore allow translational and rotational degrees of freedom. Such knee replacement prostheses are conventionally known as posterior stabilized knee replacement prosthesis. One example of a posterior stabilized knee replacement prosthesis is Stryker's Scorpio family of knee replacement prosthesis. U.S. Pat. No. 7,160,330 describes a posterior stabilized knee replacement prosthesis that has a articular surface at least a portion of which has constant radius. U.S. Pat. No. 7,160,330 is incorporated herein by reference in its entirety. Since these posterior stabilized systems mimic natural knee kinematics, they do not create excessive forces at the bone-implant interface. However, posterior stabilized knee replacement prosthesis can get dislocated. Therefore, often, if additional stability is required over that afforded by a design with a post-cam articulation, a hinged knee may be required.
Thus, there exists a need for a total knee replacement design that offers the benefits of both a hinged knee (stability despite lack of soft tissue constraint) and a non-hinged knee (natural knee kinematics).
As used herein, when referring to bones or other parts of the body, the term “proximal” means closer to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means towards the head. The term “anterior” means towards the front part of the body or the face and the term “posterior” means towards the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body.